Chronic ischemic heart disease in humans often is associated with ventricular tachycardia that may be caused by reentrant excitation. In this application we propose to investigate reentrant excitation in canine hearts with subacute and chronic infarction to define the anatomical and electrophysiological properties of the reentrant circuits. To do this we have developed a system which enables us to record electrograms from 200 sites simultaneously on the heart. Isochronal mapping is accomplished by computer assisted analysis of activation times so that we can "follow" impulse propagation during arrhythmias. Reentrant tachycardia will be induced by programmed stimulation of the ventricles in dogs with 1 week - 2 month old myocardial infarcts. Reentrant circuits will be located by mapping and their anatomy defined (histology and ultrastructure). Anatomical characteristics will be correlated with electrocardiographic features of the arrhythmia e.g., do large or small circuits cause nonsustained, or sustained tachycardia, and what are the anatomical features of circuits associated with tachycardias that degenerate into fibrillation? When sustained tachycardia occurs we will determine the mechanism by which ventricular stimulation can terminate it, by mapping stimulated and reentrant impulses. Protocols are described to determine whether the slow conduction and block which leads to reentry are caused by tissue anisotropy, abnormalities of the transmembrane potentials of myocardial fibers in the reentrant circuit or a combination of both. To do this transmembrane potentials will be recorded from the in situ heart. Comparisons will be made between mechanisms causing reentry in infarcts less than 1 week old with those 2 months old since preliminary data shows that there is a marked change in the anatomy and electrophysiology of reentrant circuits as infarcts age. Finally, the anatomical and electrophysiological cause of fractionated electrograms recorded from chronic infarcts will be defined since it has been proposed that the location of such electrograms may indicate the site of reentrant circuits in humans.